Abstract
Thermal decomposition of bibenzyl has been studied both in liquid and in gas phases at temperatures ranging from 360 to 460°C and in the presence of 1 to 15 parts of tetralin as a hydrogen donating agent.
Reactions in liquid-phase were carried out in a high pressure autoclave of 200ml in volume equipped with a stirring device under pressures ranging from 50 to 80 kg/cm2•G. The product, as analyzed by gas chromatography, consisted mainly of toluene, naphthalene and hydrogen in amounts consistent with the following stoichiometry, (Table 1, Fig. 1)
(1)
Included also in the product were small amounts of 1-methylindane and to a still lesser extent of n-butylbenzene, both of which had been identified as derived from tetralin but not from bibenzyl5), 6)
Gas-phase reaction was carried out under the pressure of about 100torr in a 100ml glass vessel connected to an evacuation unit for conventional handlings of both reactants and products. The main products and their distributions were again in conformity with the stoichiometry given above. (Table 3, Fig. 5) Contrary to the liquid-phase reactions, however, traces of benzene, ethylbenzene and styrene were the contaminants of gas-phase reaction products. At higher levels of conversion and lower tetralin to bibenzyl molar ratios, trans-stilben and phenanthrene were also detected.
Except for liquid-phase data obtained with molar ratios of tetralin to bibenzyl of less than 9, the available data indicated that the first-order rate law was obeyed and that the rate constant tended to increase with the ratio of tetralin (Figs. 2, 3, 6). The values of the rate constant were well fitted with the Arrhenius formulation. The values of activation energy and A-factor were thus deduced as being 61.5kcal/mol and 1014.4sec-1 in the liquidphase, and 60.4kcal/mol and 1014.8sec-1 in the gas-phase. (Tables 2, 4, Fig. 4) The following sequence of consecutive reactions has been proposed to account for the present findings.
(2)
(3)
(4)
In terms of the proposed scheme and assuming reaction (2) to be the rate-controlling step, the resonance stabilization energy of benzyl radical has been estimated to be approximately 13kcal/mol in good agreement with 0.72β deduced from the simple Hückel calculation. Relevant reports both in accord and in disaccord with the present results have also been discussed7)-9). (Tables 5, 6)
